1. Field of the Invention
The present invention relates to an optical head device and an optical information recording or reproducing device for performing recording or reproducing on optical recording medium. In particular, the present invention relates to an optical head device and an optical information recording or reproducing device capable of obtaining fine track error signals and lens position signals even though there are lens shifts, with respect to plural types of optical recording medium having different pitches of grooves.
2. Related Art
In write-once and rewritable optical recording medium, grooves for tracking are formed generally. When detecting tracking error signals with respect to these optical recording medium, detections are typically performed by means of a push-pull method. However, a tracking error signal detected by the push-pull method generates an offset when an objective lens of the optical head device shifts in a radial direction of the optical recording medium. In order to prevent deterioration in recording or reproducing characteristics due to the offset caused by such a lens shift, an optical head device and an optical information recording or reproducing device are required to be devised so as not to generate an offset caused by a lens shift in a track error signal.
On the other hand, when an optical head device performs a track follow operation to an optical recording medium, an objective lens of the optical head device follows a track of the optical recording medium in accordance with a track error signal, and the optical system of the optical head device, except for the objective lens, follows the objective lens such that the objective lens will not be shifted from a mechanically neutral position with respect to the optical system of the optical head device except for the objective lens. Further, when the optical head device performs a seek operation to the optical recording medium, in general, the objective lens is fixed at a mechanically neutral position with respect to the optical system of the optical head device except for the objective lens, and the optical system of the optical head device except for the objective lens moves in a radial direction of the optical recording medium corresponding to a seek signal. In order to perform such a track follow operation and a seek operation stably, an optical head device and an optical information recording or reproducing device are required to be devised so as to detect a lens position signal indicating a shifted amount from the mechanically neutral position of the optical lens.
In general, when viewed from the side of an incident light to an optical recording medium, dented parts of grooves formed in the optical recording medium are called “grooves”, and protruded parts are called “lands”. Write-once and rewritable optical recording medium include: optical recording medium of a groove recording system such as DVD-R (Digital Versatile Disc-Recordable) and DVD-RW (Digital Versatile Disc-Rerecordable) in which recording or reproducing is performed only to grooves; and optical recording medium of a land/groove recording system such as DVD-RAM (Digital Versatile Disc-Rewritable) in which recording or reproducing is performed to both lands and grooves. Typically, the pitches of grooves in optical recording medium of the groove recording system are narrower than the pitches of grooves in optical recording medium of the land/groove recording system. Optical head devices and optical information recording or reproducing devices are required to be devised so as to cope with a plurality of optical recording medium having different pitches of grooves.
As an optical head device capable of detecting a lens position signal while not generating an offset due to a lens shift in a tracking error signal with respect to plural types of optical recording medium with different pitches of grooves, there may be one described in Japanese Patent Application Laid-open No. 9-81942 (Patent Document 1).
FIG. 1 shows the configuration of an optical head device described in Patent Document 1.
Light emitted from a semiconductor laser 1 is collimated by a collimator lens 2. The collimated light is then divided into three light beams by a diffractive optical element 3e which are 0th-order light as a main beam and ± 1st-order diffracted lights as sub-beams. The light beams enter a polarizing beam splitter 4 as P polarized light and substantially 100% transmit therethrough. Then, they transmit through a quarter-wave plate 5 to be converted from linearly polarized light to circularly polarized light thereby to be focused onto a disk 7 by an objective lens 6. Three light beams reflected from the disk 7 transmit inversely through the objective lens 6 and then transmit the quarter-wave plate 5 to be converted from the circularly polarized light to linearly polarized light whose polarization direction is orthogonal to that in the outward path. The light beams then enter the polarizing beam splitter 4 as S polarized light and substantially 100% is reflected thereby to be received by a photodetector 10 through a cylindrical lens 8 and lens 9. The photodetector 10 is placed between the two focal lines of the cylindrical lens 8 and lens 9.
FIG. 2 is a plan view of the diffractive optical element 3e. The diffractive optical element 3e is so configured as to include a diffraction grating divided into two areas 15a and 15b by a line passing through the optical axis of an incident light and in parallel with a direction corresponding to a tangential direction of the disk 7. The directions of the lattices in the diffraction gratings are in parallel with the direction corresponding to the radial direction of the disk 7 and its pattern is in a linear form at even intervals. In the areas 15a and 15b, intervals of the lattices are equal. The phase of the lattice in the area 15a and the phase of the lattice in the area 15b are shifted by π to each other. Note that a dotted line in FIG. 2 shows the effective diameter of the objective lens 6.
Now, it is assumed that the wavelength of the semiconductor laser 1 is λ, the diffraction rate of the lattice is n, the height of the lattice is h, and h=0.115λ/(n−1). Here, the transmission factor of the lattice is 87.5%, and ± 1st-order diffraction efficiency is about 5.1%, respectively. That is, the lights made incident on the areas 15a and 15b are transmitted about 87.5% as a 0th-order light, and are diffracted about 5.1% as ± 1st-order diffracted lights, respectively. The phase of the +1st-order diffracted light from the area 15a and the phase of the +1st-order diffracted light from the area 15b are shifted by n to each other. Similarly, the phase of the −1st-order diffracted light from the area 15a and the phase of the −1st-order diffracted light from the area 15b are shifted by π to each other.
On the disk 7, there are formed three focused spots, corresponding to the 0th-order light, the +1st-order diffracted light and the −1st-order diffracted light from the diffractive optical element 3e. The three focused spots are located on the same track on the disk 7.
FIG. 3 shows the pattern of the light receiving sections of the photodetector 10 and the configuration of the light spots on the photodetector 10. A light spot 16a corresponds to 0th-order light from the diffractive optical element 3e, and is received by four divided light receiving sections 17a to 17d which are divided by division line in parallel with the direction corresponding to the tangential direction of the disk passing through the optical axis and the division line in parallel with the direction corresponding to the radial direction. A light spot 16b corresponds to +1st-order diffracted light from the diffractive optical element 3e, and is received by four divided light receiving sections 17e to 17h which are divided by division line in parallel with the direction corresponding to the tangential direction of the disk passing through the optical axis and the division line in parallel with the direction corresponding to the radial direction. A light spot 16c corresponds to −1st-order diffracted light from the diffractive optical element 3e, and is received by four divided light receiving sections 17i to 17l which are divided by division line in parallel with the direction corresponding to the tangential direction of the disk passing through the optical axis and the division line in parallel with the direction corresponding to the radial direction. The row of the three focused spots on the disk 7 is in the tangential direction, however, the row of the light spots 16a, 16b, 16c on the photodetector 10 is in the radial direction due to the effect of the cylindrical lens 8 and the lens 9.
When outputs from the light receiving sections 17a to 171 are indicated as V17a to V17l, respectively, a focus error signal is obtained from a calculation of FE=(V17a+V17d)−(V17b+V17c) by means of an astigmatism method. A push-pull signal of the light spot 16a, or the main beam, is obtained from a calculation of PPM=(V17a+V17b)−(V17c+V17d). A Push-pull signal of the optical spots 16b and 16c, or the sub-beams, is obtained from a calculation of PPS=(V17e+V17f+V17i+V17j)−(V17g+V17h+V17k+V17l). A track error signal by means of a differential push-pull method is obtained from a calculation of TE=PPM−α*PPS (α is a constant). A lens position signal is obtained from a calculation of LP=PPM+β*PPS (β is a constant). Further, an RF signal is obtained from a calculation of RF=V17a+V17b+V17c+V17d. 
FIG. 4 shows phases of a sub-beam reflected from the disk 7 and of sub-beams diffracted from the disk 7 in a case of using a disk of groove recording system with a narrow pitch of grove as the disk 7. It is assumed that focused spots, which are sub-beams, are located at the center of the track of the disk 7. Areas 24a and 24b correspond to diffracted lights, diffracted from the areas 15a and 15b of the diffractive optical element 3e respectively, of the sub-beam reflected from the disk 7 as the 0th-order light. Areas 25a and 25b correspond to diffracted lights, diffracted from the areas 15a and 15b of the diffractive optical element 3e respectively, of the sub-beam diffracted from the disk 7 as the +1st-order diffraction light. Areas 26a and 26b correspond to diffracted lights, diffracted from the areas 15a and 15b of the diffractive optical element 3e respectively, of the sub-beam diffracted from the disk 7 as the −1st-order diffracted light. In an area indicated as + and an area indicated as −, phases of the lights are shifted by π to each other. Note that dotted lines shown in FIG. 4 indicate the effective diameter of the optical lens 6.
In FIG. 4, R indicates an effective radius of the objective lens 6, and D1 indicates a distance between the centers of the sub-beam reflected from the disk 7 and of the sub-beam diffracted from the disk 7 on the pupil face of the objective lens 6. Assuming that the wavelength of the semiconductor laser 1 is λ, the focal length of the objective lens 6 is f, the numerical aperture in the objective lens 6 is NA, the pitch of a grove in the disk 7 of groove recording system is Tp1, R=f*NA and D1=f*λ/Tp1 are established. Here, assuming that λ=405 nm, f=3.05 mm, NA=0.65 and Tp1=0.4 μm, the following results are obtained: R=1.98 mm and D1=3.09 mm.
FIG. 5 shows phases of a sub-beam reflected from the disk 7 and of sub-beams diffracted from the disk 7 in a case of using a disk of land/groove recording system with a wide pitch of groove as the disk 7. It is assumed that focused spots, which are sub-beams, are located at the center of the track of the disk 7. Areas 24a and 24b correspond to diffracted lights, diffracted from the areas 15a and 15b of the diffractive optical element 3e respectively, of the sub-beam reflected from the disk 7 as the 0th-order light. Areas 25a and 25b correspond to diffracted lights, diffracted from the areas 15a and 15b of the diffractive optical element 3e respectively, of the sub-beam diffracted from the disk 7 as the +1st-order diffracted light. Areas 26a and 26b correspond to diffracted lights, diffracted from the areas 15a and 15b of the diffractive optical element 3e respectively, of the sub-beam diffracted from the disk 7 as the −1st-order diffracted light. In an area indicated as + and an area indicated as −, the phases of lights are shifted by π to each other. Note that dotted lines shown in FIG. 4 indicate the effective diameter of the optical lens 6.
In FIG. 5, R indicates the effective radius of the objective lens 6, and D2 indicates a distance between the centers of the sub-beam reflected from the disk 7 and of the sub-beam diffracted from the disk 7 on the pupil face of the objective lens 6. Assuming that the wavelength of the semiconductor laser 1 is λ, the focal length of the objective lens 6 is f, the numerical aperture in the objective lens 6 is NA, the pitch of a groove in the disk 7 of land/groove recording system is Tp2, R=f*NA and D2=f*λ/Tp2 are established. Here, assuming that λ=405 nm, f=3.05 mm, NA=0.65 and Tp2=0.68 μm, the following results are obtained: R=1.98 mm and D2=1.82 mm.
A push-pull signal is detected by using the fact that a light reflected from the disk 7 and the light diffracted from the disk 7 are interfered with each other at a part where both are overlapped, and the intensities of the lights interfered are changed by respective phases. In the part where the sub-beam reflected from the disk 7 and the sub-beam diffracted from the disk 7 are overlapped, if the phases of the both beams are shifted by π to each other, a push-pull signal of the sub-beam has the inverse polarity to a push-pull signal of the main beam. In contrast, if the phases of the both sub-beams coincide with each other, a push-pull signal of the sub-beam has the polarity the same as that of a push-pull signal of the main beam.
FIGS. 6(a) and 6(b) show calculation examples of the relationships between the off-track amount and push-pull signals in the case of using a disk of groove recording system with a narrow pitch as the disk 7. As conditions of calculation, the groove depth of the disk 7 is set to 0.1 λ, in addition to the conditions described in FIG. 4. FIG. 6(a) shows the relationship between the off-track amount and a push-pull signal of the main beam, and FIG. 6(b) shows the relationship between the off-track amount and a push-pull signal of the sub-beam. Further, white circles show the calculation results in a case of the lens shift amount being 0 μm, gray circles show the calculation results in a case of the lens shift amount being 100 μm, and black circles show the calculation results in a case of the lens shift amount being 200 μm. The horizontal axes of the Figures are standardized by the pitch of a groove of the disk 7, and the vertical axes thereof are standardized by the level of a sum signal in the case where there is no groove in the disk 7.
In FIG. 4, the area 24a of the 0th-order light and the area 25b of the +1st-order diffracted light are overlapped, and the area 24b of the 0th-order light and the area 26a of the −1st-order diffracted light are overlapped. In the area 24a and the area 25b, phases of the lights are shifted by π to each other, and in the area 24b and the area 26a, phases of the lights are shifted by π to each other. Therefore, a push-pull signal of the sub-beam has the inverse polarity to that of a push-pull signal of the main beam.
If there is a lens shift, the boundary between the area 24a and the area 24b, the boundary between the area 25a and the area 25b, and the boundary between the area 26a and the area 26b are shifted to the left side or the right side of FIG. 4 according to the direction of the lens shift. However, assuming that the maximum value of the lens shift amount is 200 μm, the width of the part where the area 24a of the 0th-order light and the area 25b of the +1st-order diffracted light are overlapped remains unchanged, and the width of the part where the area 24b of the 0th-order light and the area 26a of the −1st-order diffracted light are overlapped remains unchanged.
Due to the reasons described above, a push-pull signal of the sub-beam has the inverse polarity to that of a push-pull signal of the main beam, which is obvious by comparing FIG. 6(a) with FIG. 6(b). Further, the amplitude of a push-pull signal of the sub-beam is the same as that of a push-pull signal of the main beam, not depending on the amount of lens shift.
On the other hand, in either FIG. 6(a) or FIG. 6(b), offsets of the same sign are generated in push-pull signals if there is a lens shift, and the offset amount increases as the lens shift amount increases. Although the sign of the offset is negative here, the sign of the offset is changed to positive if the direction of the lens shift is reversed.
FIGS. 7(a) and 7(b) show calculation examples of the relationships between the off-track amount and a tracking error signal and between the off-track amount and a lens position signal in the case of using a disk of groove recording system with a narrow pitch of groove as the disk 7. The conditions of calculation are the same as those described for FIGS. 6(a) and 6(b). FIG. 7(a) shows the relationship between the off-track amount and a track error signal by means of a differential push-pull method, and FIG. 7(b) shows the relationship between the off-tack amount and a lens position signal. Further, white circles indicate calculation results in the case of the lens shift amount being 0 μm, gray circles indicate calculation results in the case of the lens shift amount being 100 μm, and black circles indicate calculation results in the case of the lens shift amount being 200 μm.
The horizontal axes in the Figures are standardized by the pitch of a groove of the disk 7. Assuming that sum signals of the main beams and the sub-beams are SUMM and SUMS, respectively, the vertical axes of FIGS. 6(a) and 6(b) are given by PPM/SUMM and PPS/SUMS, respectively. Here, assuming that α=β=SUMM/SUMS, the vertical axis of FIG. 7(a) is (PPM/SUMM−PPS/SUMS)/2=TE/(SUMM+α*SUMS), and the vertical axis of FIG. 7(b) is (PPM/SUMM+PPS/SUMS)/2=LP/(SUMM+β*SUMS). That is, the vertical axis of FIG. 7(a) is standardized by SUMM+α*SUMS, and the vertical axis of FIG. 7(b) is standardized by SUMM+β*SUMS.
In FIG. 7(a), an offset of a push-pull signal due to a lens shift is canceled by obtaining the difference between a push-pull signal of the main beam and a push-pull signal of the sub-beam, whereby a fine track error signal not causing an offset is obtained even though there is a lens shift. Further, the amplitude of the track error signal is constant, not depending on the lens shift amount.
On the other hand, in FIG. 7(b), a component (groove crossing noise) which varies depending on the off-track amount is canceled by obtaining the sum of a push-pull signal of the main beam and a push-pull signal of the sub-beam, not depending on the lens shift amount, so that a fine lens position signal without a groove crossing noise is obtained.
FIGS. 8(a) and 8(b) show calculation examples of the relationships between the off-track amount and push-pull signals in the case of using a disk of land/groove recording system with a wide pitch of groove as the disk 7. As the calculation conditions, the groove depth of the disk 7 is set to 0.18 λ, in addition to the conditions described for FIG. 5. FIG. 8(a) shows the relationship between the off-track amount and a push-pull signal of the main beam, and FIG. 8(b) shows the relationship between the off-track amount and a push-pull signal of the sub-beam. Further, white circles show the calculation results in the case of the lens shift amount being 0 μm, gray circles show the calculation results in the case of the lens shift amount being 100 μm, and black circles show the calculation results in the case of the lens shift amount being 200 μm. The horizontal axes of the Figures are standardized by the pitch of a groove of the disk 7, and the vertical axes thereof are standardized by the level of a sum signal in the case where there is no groove in the disk 7.
In FIG. 5, the area 24a of the 0th-order light and the area 25b of the +1st-order diffracted light are almost overlapped, and the area 24b of the 0th-order light and the area 26a of the −1st-order diffracted light are almost overlapped. In the area 24a and the area 25b, phases of the lights are shifted by π to each other, and in the area 24b and the area 26a, phases of the lights are shifted by π to each other. Therefore, a push-pull signal of the sub-beam obtained in this area has the inverse polarity to that of a push-pull signal of the main beam. However, in FIG. 5, the area 24a of the 0th-order light and the area 25a of the +1st-order diffracted light, and the area 24b of the 0th-order light and the area 25b of the +1st-order diffracted light are also overlapped partially, respectively, and the area 24b of the 0th-order light and the area 26b of the −1st-order diffracted light, and the area 24a of the 0th-order light and the area 26a of the 1st-order diffracted light are also overlapped partially, respectively. In the area 24a and the area 25a, and in the area 24b and the area 25b, phases of the lights coincide, and further, in the area 24b and the area 26b, and in the area 24a and the area 26a, phases of the lights coincide. Therefore, push-pull signals of the sub-beams, obtained from these parts, have the same polarity to that of a push-pull signal of the main beam.
When there is a lens shift, the boundary between the area 24a and the area 24b, the boundary between the area 25a and the area 25b, and the boundary between the area 26a and the area 26b are shifted to the left side or the right side in FIG. 5 according to the direction of the lens shift. If these boundaries are shifted to the left side in FIG. 5, the width of the part where the area 24a of the 0th-order light and the area 25b of the +1st-order diffracted light are overlapped becomes narrower, and also the width of the part where the area 24b of the 0th-order light and the area 26a of the −1st-order diffracted light are overlapped becomes narrower. In contrast, the width of the part where the area 24b of the 0th-order light and the area 25b of the +1st-order diffracted light are overlapped becomes wider, and also the width of the part where the area 24b of the 0th-order light and the area 26b of the −1st-order diffracted light are overlapped becomes wider.
Due to the reasons described above, a push-pull signal of the sub-beam has the inverse polarity to that of the push-pull signal of the main beam, which is obvious by comparing FIG. 8(a) with FIG. 8(b). However, the amplitude of a push-pull signal of the sub-beam is smaller than that of a push-pull signal of the main beam, and as the lens-shift amount increases, the difference between the amplitude of a push-pull signal of the sub-beam and that of a push-pull signal of the main beam increases.
On the other hand, in either FIG. 8(a) or FIG. 8(b), offsets of the same sign are generated in push-pull signals if there is a lens shift, and the offset amount increases as the lens shift amount increases. Although the sign of the offset is negative here, the sign of the offset is changed to positive if the direction of the lens shift is reversed.
FIGS. 9(a) and 9(b) show calculation examples of the relationships between the off-track amount and a track error signal and between the off-track amount and a lens position signal in the case of using a disk of land/groove recording system with a wide pitch of groove as the disk 7. The conditions of calculation are the same as those described for FIGS. 8(a) and 8(b). FIG. 9(a) shows the relationship between the off-track amount and a track error signal by means of a differential push-pull method, and FIG. 9(b) shows the relationship between the off-tack amount and a lens position signal. Further, white circles indicate calculation results in the case of the lens shift amount being 0 μm, gray circles indicate calculation results in the case of the lens shift amount being 100 μm, and black circles indicate calculation results in the case of the lens shift amount being 200 μm.
The horizontal axes in the Figures are standardized by the pitch of a groove of the disk 7. Assuming that sum signals of the main beams and the sub-beams are SUMM and SUMS, respectively, and the vertical axes of FIGS. 8(a) and 8(b) are given by PPM/SUMM and PPS/SUMS. Here, assuming that α=β=SUMM/SUMS, the vertical axis of FIG. 9(a) is (PPM/SUMM−PPS/SUMS)/2=TE/(SUMM+α*SUMS), and the vertical axis of FIG. 9(b) is (PPM/SUMM+PPS/SUMS)/2=LP/(SUMM+β*SUMS). That is, the vertical axis of FIG. 9(a) is standardized by SUMM+α*SUMS, and the vertical axis of FIG. 9(b) is standardized by SUMM+β*SUMS.
In FIG. 9(a), an offset of a push-pull signal due to a lens shift is canceled mostly by taking the difference between a push-pull signal of the main beam and a push-pull signal of the sub-beam, whereby a fine track error signal causing little offset is obtained even though there is a lens shift. However, if there is a lens shift, the amplitude of the track error signal is reduced. Therefore, as the lens shift amount increases, the reduced amount of the amplitude also increases.
On the other hand, in FIG. 9(b), if there is no lens shift, a component (groove crossing noise) which varies depending on the off-track amount is canceled by obtaining the sum of a push-pull signal of the main beam and a push-pull signal of the sub-beam, so that a fine lens position signal without a groove crossing noise is obtained. However, if there is a lens shift, the groove crossing noise is not canceled completely and remains even when the sum of the push-pull signal of the main beam and the push-pull signal of the sub-beam is taken. Therefore, as the lens shift amount increases, the amplitude of the remaining groove crossing noise also increases. The ratio of the amplitude of the groove crossing noise to a DC component of the lens position signal is about 1.77, which is large.
As described above, in the optical head device according to Patent Document 1, it is possible to obtain a fine track error signal and a fine lens position signal even though there is a lens shift with respect to an optical recording device of a groove recording system with a narrow pitch of groove. However, with respect to an optical recording device of a land/groove recording system with a wide pitch of groove, the amplitude of a track error signal decreases, and a groove crossing noise remains in a lens position signal if there is a lens shift.